Range Report very fast twist rates??

[TOPGuN050]

i recently read an article in this years issue of sniper magazine that i found very interesting.
the article was written by Todd Hodnett and is titled The Future Of Twist Rates.

he is stating that he is getting substantial gains out of a 1:8 or 1:9 for his 338Lapua
also for his .308 he is shooting a 1:7.8 and has a friend shooting a 1:4 twist in his.

he is saying they are getting better downrange results with these faster twist rates. he also states that the reason we are not seeing these more often
is because its not the industry standard, and people will not stray from their comfort zone.

he also states that a faster twist rate helps our new and improved LR bullets with high BC really shine down range.

any thoughts and opinions on this are appreciated, thanks

i am considering a 1:8 for my 300wm after reading this, but im not much for jumping in head first after reading an article, even if its Todd Hodnett's writing.

 

[RTH1800]

Substantial gains of what? Velocity, accuracy, barrel life, bullet expansion?

 

[sab9259]

Tag for this. I have a 1:8 .308 coming

 

[bohem]

Don't confuse Hodnett's ability to shoot for a graduate engineering degree in Aeroballistics Theory.

Bullet design is FAR more important than pushing a faster twist rate.

ETA:

Let me expand on this, as I was somewhat snide and didn't give any insight into why I don't necessarily agree with him.

There is a known behavior that some bullets will shoot very repeatedly down into transonic effects, down through mach transition and for hundreds of yards after transitioning to full subsonic flight speed. Some others are miserable at this.

Case in Point:

223 Rem (5.56x45 NATO) with 55gr M193 FMJBT vs. 80gr SMK shot out of their optimally twisted barrels, 12 and 7 as well as BOTH shot from the 7tw

The M193 ammo doesn't have a prayer of transitioning the sound barrier before turning sideways and whiffling away to who knows where except under extremely special atmospheric conditions. It is quite literally a "brick wall" where the bullet hits a specific speed and pretty much falls out of the sky over the next 50-100yd.

Unless the bullet is flying at substantially higher than sea level (8-10kft+ DA) it is really worthless expenditure of ammo to try hitting something past about 500-600yd with it (again, depending on DA for effective range).

The 12tw vs. 7tw debate here has little effect because of the supremely poor design on the tail of that bullet.

The 80SMK however, is a different story. If it is fully stable at the muzzle then it is stable until it hits the ground at max range. The bullet is known to transition without issue even in several thousand feet of NEGATIVE DA. I've done it personally with my 7 and 7.5tw rifles.



Another is the comparison of a 180 Berger Hybrid and a 175SMK both in 7mm. The 9tw will stabilize both and dropping to a 1:8 does little except rob forward velocity. I have a 1:7.5tw 7mm that won't get a 180 Hybrid through the sound barrier but even with a 1:9tw the 175SMK 7mm makes hits several hundred yards past full sonic transition.



I will skip the math that dictates this behavior since you can write textbooks on the topic (see Bob McCoy's book "Modern Exterior Ballistics") but suffice it to say I do not share the same opinion as Mr. Hodnett.

 

[sinister]

A faster twist rate can help a well-designed, quality-made bullet maintain stability farther as it transitions through supersonic.

Spin (or over-spin) a poorly-made bullet too quickly and you can incur instability around its axis, and in extreme instances it'll come apart where the jacket is scored by the lands (if the jacket is too thin).

 

[Graham]

A faster twist rate can help a well-designed, quality-made bullet maintain stability farther as it transitions through supersonic.

Spin (or over-spin) a poorly-made bullet too quickly and you can incur instability around its axis, and in extreme instances it'll come apart where the jacket is scored by the lands (if the jacket is too thin).

Sinister, correct me if I'm wrong on this, but The .224 90's needed a 6.5 twist - which is about as fast as anything can be spun - and they still didn't outshine the 80's with a 7-twist. So there are practical limitations to doubling the RPMs, even with good bullets.

 

[sinister]

True on the 6.5s and 90s (they need a minimum twist rate)-- but the 6.5 twist really made the 77s shine at 200 and 300.

 

[gstaylorg]

Having used a .308 in 1000 yd F-T/R competitions for some time had that a much faster than necessary twist rate (1 in 9"), my take is that overspinning may possibly buy you something under the right situation, but F-Class comps aren't that situation. Running both Berger 175 Tactical bullets and JLK 180s through that rifle, it became painfully obvious over time that I was getting a lot more vertical dispersion than with my other (very similar) rifles that have more reasonable twist rates. I attribute at least some, if not most, of that vertical dispersion to the effect of wind using that excessive twist rate.

It's also worth noting that many of the top F-Class shooters are actually using what might be perceived as minimal, or even sub-optimal twist rates for the projectiles they're using. Twist rates such as 1 in 12" or even 1 in 13" for Berger 185s are not unheard of, and last year's F-T/R National Champion James Crofts had his latest comp gun built for 185s and 200s with an 11-twist. The math/physics certainly supports that a faster twist rate can marginally improve the ballistics of a given projectile when shooting it out to the extreme envelope of its performance, but I strongly suspect that if an extremely fast twist rate was a big advantage in terms of precision across the board, you'd find more top LR shooters using one.

 

[bohem]

gstaylorg - what you're describing is termed "aerodynamic jump" and you're right, the faster you twist the barrel the more effect wind has vertically.

 

[TOPGuN050]

thank you all for the replys, i am currently shooting a 300wm with a 1:10 twist 26in barrel.

i am getting around 3000fps out of my 190smk bullets and it shoots very well for a savage factory setup except for having the bolt head true

what i got from reading this article was that hodnett was saying, bullets that fall short at the transonic range can be spun faster hence boosting the performance past transonic. his holdovers or corrections were less and he stated that the bullet is carrying more downrange energy.

after reading your replys, i dont think i will gain all that much in going from a 1:10 to a 1:8. hodnett may disagree but if it works now and it works well, why change it?

thanks again.

 

[gstaylorg]

I played around with some calculations for the increased vertical deflection with the 9-twist barrel versus my 10- and 11-twist barrels using my specific MV and load data along with the equations in Bryan Litz' Applied Ballistics for Long Range Shooting. The calculated values were pretty close to what I actually observed with the 9-twist barrel. My conclusion would be unless you're: A) pushing a projectile to the extreme limits of its performance; or B) fortunate enough to shoot where there's no wind, I can't imagine that going with a much faster twist is going to buy much with the increased wind deflection that goes along with it. In the case of A), my inclination would simply be to use a projectile better suited to the longer range than to overspin one that was marginal.


Edited to add:TOPGuN050 - if you look in Bryan's book, there is a section on this very phenomena. Two things I would note from reading that section; first, the gains in distance by increasing spin rate before the projectiles became unstable weren't all that great; and second, it didn't appear to me as as though it would buy you much with a projectile that was inherently stable through trans-sonic. So in part, the choice of a better projectile could substantially mitigate the need to overspin it.

 

[sinister]

Todd's emphasis is aimed at trying to get optimum maximum range performance from the shortest barrel possible. He likes 20-inch barrels with a fast twist while using a 300-grain bullet. The twist helps maintain stability as the bullet goes marginal, trans-sonic, then subsonic.

A Palma shooter pushing a 155-grain .308 may choose the slowest twist possible (1-13 is very popular) while gaining the maximum velocity from a 28 to 30-inch barrel. The X- and 10-ring values for the NRA and Commonwealth Palma targets, shot with iron sights, are not very forgiving. Any vertical error induced by wind (higher and right or lower and left, depending on whether it's a left or right cross-wind) can knock you down a point or two -- something no one wants in smack-talk competition that only comes around once every four years.

 

[VKC]

Theoretically a faster twist rate will help you maintain bullet stability in transonic range. However, at closer ranges you may notice less accuracy.

A faster twist rate will give you an advantage in stabilizing heavy subsonic suppressed rounds in a shorter barrel.

 

[TOPGuN050]

i guess my decision is made for when i rebarrel my 300. i am thinking a 28in. 1:10. those extra 2 inches may help to add a little bit of weight out front.

 

[Greg Langelius *]

I prefer to use the slowest twist rate that will unquestionably stabilize a given projectile. RPM decays more slowly than Velocity, which equates to me as causing the bullet to become more statically (centrifugal) stable, and less dynamically (aerodynamic) stable as flight time increases.

As dynamic stability decreases, drag increases, proportionally decreasing effective BC.

This theory has shown some small pragmatic confirmation when using bullets that are fired in barrels that have excessive(?) twist (120NBT in .260 1:8" twist at 3000+fps, fired at 1000yd); i.e. drop at extreme range far exceeds that which was predicted by ballistic computation. Targets showed a consistently nose-up impact, several feet below predicted POI; yet dispersal was not remarkably greater than 140/142's.

This observation is anecdotal, and is based on too small a sample to be taken conclusively; but it does seem to bear out my proposition to at least some small degree.

Other effects, like Magnus Effect, Spin Drift(?), etc. would also seem to be aggravated by additional spin, but for the conditions I shoot, the differences would most likely be negligible.

Greg

 

[bodywerks]

What gstaylorg and Greg said. Over spinning a projectile had the potential to to more harm than good. A projectile had the least amount if spin stability right as it leaves the muzzle. Soon stability factor only gets better from there.
Short range Benchrest shooters routinely choose the absolute lowest twist rate barrel that will stabilize their bullet for a reason - less aerodynamic jump, spin drift and coning.
I see zero advantage to having an overly fast twist rate.

 

[Nortex]

I'm in the camp as the 2 above as I use a slower twist than most. My 7mm has a 1:10" twist and does just fine with 162's.

 

[damoncali]

I wrote an article on this. In a nutshell, fast twist rates are bad. My .30BR is a 1:18. Shoots great.

Barrel Twist & Bullet Stability | Bison Ballistics

 

[goodgorilla]

When is does a twist rate become fast? Say if we have a slowest twist rate of 10, when does the twist rate become fast? 8?

 

[BryanLitz]

As a rule of thumb, you could say that 35 calibers/turn is normal, and 30 calibers/turn is fast.

For example, a .308 barrel that's 35 calibers/turn is .308*35 = 10.8" twist.

A .338 barrel that's 35 calibers/turn is .338*35 = 11.8"

35 calibers per turn is what you'll typically find, as a rule of thumb, in many factory rifles.

Going to 30 calibers/turn is:
.224 caliber: .224*30 = 6.7"
.308 caliber: .308*30 = 9.2"
.338 caliber: .338*30 = 10.1"

Again, this is a rule of thumb generalization.

You'll notice that some calibers like .257 and .277 have traditionally slow twist rates, while others like .264 (6.5mm) and .284 (7mm) traditionally have faster twist rates.

The tendency for a caliber to have fast twist rates allows for longer, heavier, higher BC bullets to be stabilized, which makes those calibers more suitable for long range. This is why you see a lot of 6.5mm and 7mm long range guns, while not so many .257 and .277 caliber. The calibers are only a few thousandths different in size, but the twist rate determines the suitability for long range.

Take care,
-Bryan

 

[Struggles]

Tag for this. I have a 1:8 .308 coming

Very curious what kind of results you found with this barrel, any accuracy gains out at longer distances?

 

[HAZORD]

By the way, he didn't mention it in his post, but Bryan Litz covers this very subject at length in his latest book "Modern Advancements in Long Range Shooting Vol. 1" proving it with examples of testing different twist rates and how the tighter twist improves BC.